US2988575A - Production of isomeric alkyl benzenes - Google Patents

Description

June 13, 1961 G. E. HAYs ETAL 2,988,575

PRODUCTION oF Ism/ERIC ALKYL BENZENES Filed June l, 1954 G. E. HAYS H. M. HAWKINS ATTORNEYS ,Unird Se@ Pet@ O 2,988,575 PRODUCTION F ISOMERIC ALKYL BENZENES George E. Hays and Harold M. Hawkins, Bartlesville,

Okla., assignors to Phillips Petroleum Company, a corporation of Delaware Filed June 1, 1954, Ser. No. 433,549 1 Claim. (Cl. 260-66S) This invention relates to the production of isomeric alkyl benzenes. In one of its aspects, the invention relates to the production of para-xylene. In another of its aspects, the invention relates to the production of paraxylene from a mixture of orthoand meta-xylenes. In another of its aspects, the invention relates to the production of xylenes from a naphthenic hydrocarbon stream.

Para-xylene has for some time been an article of commerce. Hitherto, para-xylene was obtained from 'petroleum and coal tar xylene fractions, normally consis'ting essentially of ortho, metaand para-xylene and ethyl benzene, the content of the para-xylene -varying between and 20 percent.v More recently, thedevelopment of synthetic fabrics, such as Dacron, has iricreased the demand for paraxylene to such an extent that the supply of naturally occurring xylenes is insucient. Thus, considerable interest has been focused upon the preferential conversion of other hydrocarbons to paraxylene. Y

It has been known that the isomeric forms of xylene, namely the ortho-xylene, meta-xylene and para-xylene, can be converted oneinto the; other by 'somerization However, that somerization hasbeen diii'cultto'accomplish and when the somerization has beenpoaredout under conditions such as would givearelatively selective and clean cut somerization, the reaction has takenplace at such a slow rate as to makeit impractical` for com mercial operation. Even when the isomeriza tion rwas carried out under relatively drastic conditions leading to appreciable degradation of theproduct, the reaction was quite slow. The xylenes have been isomerized by purely thermal means and by treatment under certainvconditions with Friedel-Crafts type catalysts, A e.g.,' aluminum chloride plus hydrogen chloride, but the required conditions were severe, the reaction was slowand theyields were poor. A I

Further development of the somerization of xylenes resulted in the use of clay type cracking catalysts. Preferred catalysts of that type have been the acid treated clays or synthetic materials composed largely of silica in combination with alumina, magnesia, zirconia or boric oxide.

A naphtha fraction boiling generally in the range of 150 to 450 F., preferably in the range of 220 to 300 AF., is hydroformed in the presence of a supported reforming catalyst. Catalysts of the type which We prefer to use in the hydroforming chamber include such hydroforming catalysts as hydrogen fluoride treated alumina impregnated with minor portions of platinum and halogen or with molybdenum oxide, nickel, or a mixture of cobalt oxide and molybdenum oxide, and silica-alumina impregnated with like materials.

We prefer to use a hydrogen uoride treated alumina catalyst impregnated with between 0.01 and 5 Weight per- .temperature in the range of 700 F. to 1000" F., preferably 800 F. to 950 F. A pressure within the range ICC 2 of atmospheric to 1000 p.s.i.g., preferably 200 to 600 p.s.i.g., is utilized to obtain the selective formation-of xylenes while operating at a liquid hourly space velocity of 0.3 -to 10, preferably 0.5 to 6. A hydrogen to hydro'- ca-.rbon mol ratio of between 0.5:1 to 20il,preferably 1:1 to 10:1, isutilized. Each of the following objects will be attained by the aspects of this invention. i

An object of tis invention is to provide an improved process for the production of isomeric alkyl benzenes. Another object of the invention is to provide -an im proved process for the production of para-xylene. Another object of the invention is to prevent a build-up of paranic and naphthenic impurities in a system selectively producing selected isomeric alkyl benzenes. Another object of the invention is to provide a process for the production of para-xylene from a mixture of orthoand meta-xylene. Another object of the invention is to provide a method for the production of ortho-xylene from mixtures of isomeric ,forms of xylene. `Another object of the invention is to provide a method for the somerization of ortho- Yand meta-xylenes in the Vpresence of a specific silica-alumina catalyst to selectively produce para-xylene. Another object of the invention is to produce para-xylene from a selected fraction of naphthenic hydrocarbons. Other, and further objects of this invention will be apparent to those skilled in the -art upon study of the accompanyingdisclosure. j Broadly speaking, this invention comprisesV the selective production of individual xylenes from anisorneric alkyl benzene mixture, from which para-xylene is removed by a separation such as fractional crystallizatiomjafter which the remaining portion of the isomeric alkyl benzene mixture i s subjected to somerization. The products of that somerization are then passed to a separationzone, A small portion of those products of isomerizatiotl, i'.e, in the neighborhood of from 8 to 20percent thereof, are subiected to solvent extraction, such as in a conventional Udex process, prior to the separation so asto remove therefrom any parains, naphthenes vor undesired aromatics which are formed during the somerization stage. The xylene fraction comprising meta, orthoand Aparajxylene is then returned to the crystal purification zone lf o'r recovery of para-xylene by fractional crystallization. Operation inthis manner has the advantage of continuously removing a suicient amount of the parafnic product so that a minimum of parans, naphthenes or undesired aromatics are supplied to the crystal purification zone.

Better understanding of this invention will be obtained upon reference to the drawing which is a schematic flow diagram used in the process of this invention.

Referring particularly to the drawing, a naphthenic hydrocarbon stock boiling in the range F. to 300 F., preferably 220 F. to 300 F., is fed through inlet conduit 11 to hydroformirng zone 12, together with hydrogen in a mol ratio of hydrogen to hydrocarbon as set forth above. The normal xylenes formers or C8 naph- -thenes, and minor amounts of ethyl benzene, are contained in the fraction boiling ythe range of 220 to 275 F. and naturally occurring xylenes are found in the fraction boiling within the range of 275 F. to-3009 F. Thus, the product which is removed from hydroforming zone 12 through conduit 13 contains isomeric alkyl benzene product, together with naturally occurring alkyl benzenes found in the original feed and higher and lower boiling materials. This product stream -is passed through a solvent extraction zone 14, together with a solvent introduced through conduit 15, which solvent, more fully discussed hereinafter, selectively rejects any parains or naphthenes present in that stream.

The ranate containing any parain or naphthene impprities is removed 'om solvent extraction zone 14 through outlet conduit 16. The extract containing the desired isomeric alkyl benzenes and any other aromatic material is passed from extraction zone 14 through conduit 17 to separation zone 18 wherein the solvent is recovered and is returned to the upper portion of extraction zone 14 through conduit 19. The aromatic material released from the solvent, together with any water released from the solvent, is removed from separation zone 18 through conduit 21 and is passed to condenser 2'. wherein any Water is condensed and is retluxed to separation zone 18 through conduit 23. The aromatic hydrocarbons are removed from condenser 22 through conduit 24 and are introduced into separation zone 25 through conduit 26.

Hydrocarbon materials, which are lower boiling than the desired xylene fraction, are removed from separation zone 25 through outlet conduit 27. Hydrocarbons which are higher boiling than the desired aromatic fraction are removed through conduit 28 as heavy products. An aromatic fraction containing ethyl benzene, ortho-xylene, meta-xylene and para-xylene, together with a small amount of toluene, and minor amounts of C8 and C9 parains, C9 aromatics and C8 naphthenes, is removed from separation zone 25, as an intermediate fraction, through conduit 29. If desired, the ortho-xylene can be removed as a second intermediate fraction through conduit 31. The fraction which is removed through conduit 29 is passed to a purification zone 32 in which paraxylene is separated from the liquid mixture. The purication which is carried out in purification zone 32 is preferably done by fractional crystallization.

The separation of para-xylene from other isomeric alkyl benzenes, by means of fractional crystallization, is becoming well known in the art, which is particularly exemplified by the processes disclosed in U.S. Patents Re. 23,810 of Schmidt and 2,540,977 of P. M. Arnold. It is preferred that the crystallization be carried out under such conditions that the crystals are continuously subjected to an internal reux of relatively pure paraxylene. Para-xylene product which is obtained from puritication zone 32 through conduit 33 usually has a purity upwards of 98 percent, though, if desired, lower purity product can be obtained. Uncrystallized material is removed from purification zone 32 through conduit 34 and is introduced, either with or without hydrogen from conduit 35, into isomerization zone 36.

If desired, we can obtain the isomerization of this hydrocarbon stream in isomerization zone 36 without the presence of extraneous hydrogen. The hydrogen to hydrocarbon mol ratio used in isomerization zone 36 is generally in the range of 0 to 20:1. The catalyst which we use in isomerization zone 36 can be any one of the isomerization catalysts which will isomerize the hydrocarbon stream to form additional para-xylene. lf desired, we can use the same catalyst as that used in hydroforming zone 12 or we can use a silica-alumina catalyst, such as .is prepared by the method of McKinney in U.S. Patents 2,142,324 and 2,147,985. In general, these catalysts are prepared by rst forming a hydrous silica gel or jelly from an alkali-silicate and an acid` washing soluble material from the gel, treating or activating the gel with an aqueous solution of a suitable alminum salt, and subsequently washing and drying -the treated material. In this manner, a part of the aluminum, presumably in the form of a hydrous oxide or loose hydroxide compound formed by hydrolysis, is selectively adsorbed bythe hydrous silica, and is not removed by subsequent washing. This selective adsorption is attested by a decrease in the aluminum content of the activating solution as Well as a decrease in pH as the activation progresses. The most often used catalyst of this type, at present, is a silica-alumina catalyst, prepared by treating a wet or partially dried hydrous silica gel with an aluminum salt solution, such as a solution of aluminum chloride or sulfate, and subsequently Washingrand drying the treated material. Whether prepared by this method or by some modiiication thereof, the catalyst will contain a major portion of silica, and a minor portion of aluminum oxide. This minor portion of alumina will generally not be in excess of 10 percent by weight, and will more often, and generally more preferably, be between about 0.1 and 1.5 or 2 percent by weight, on the dry basis.

In the above-outlined procedure, the starting materials are usually chosen from the water-soluble silicates and the commercially available mineral acids. Sulfuric and hydrochloric acids are preferred on economic grounds, although any acid may `be used which will provide suitable hydrogen ion concentration and form a silica hydrogel of proper consistency. Thus, phosphoric, acetic, nitric, and boric acids may be used in certain instances. The gel formed should be acidic and should be partially dried and Washed free of excess acid prior to activation, and the extent of drying is carefully controlled since the eventual catalyst activity is apparently somewhat de pendent on the maintenance of the hydrous oxide cor:- position prior to the activation treatment. The salt solution for activation may be prepared from any water-soluble hydrolyzable salt of aluminum, with the sulfate or chloride being preferred. Other alternate salts include acetates and nitrates. The adsorption of the hydrous aluminum oxide by the silica gel proceeds smoothly with hydrated silica gel, whereas with dried silica, the adsorption and the activation may be much less satisfactory. Active catalysts are preferably rinsed free of the salt solution and a moderate concentration elect or curing may be obtained by partial drying of the rinsed gel. The nal washing then serves to remove unadsorbed salts and free acid, and the final drying which is performed at moderate temperatures produces hard, brittle granules of gel containing negligible quantities of compounds other than silica and alumina.

Modifications may be made in the foregoing procedure and catalysts of suitable activity may result. One ob vious alternative is the addition of the aluminum salt to the silicate before gelation. This method enables the incorporation of greater proportions of aluminum oxide, but activity may not be proportional to increasing aluminum oxide contents above about l to about l5 weight percent so that 'little is gained by the modification and the proper degree of salt and acid removal may be more diicult. Non-uniform materials usually result from the mechanical mixing of hydrous aluminum oxide and silica gels, so that catalysts prepared in this manner may be less satisfactory. Other means of accomplishing the preparation may 'be devised, however, in view of the fore going description.

As indicated above, the finished gel-type catalysts comprise essentially silica and alumina, 4with various quanti ties of water. The aluminum oxide may be present in minor activating quantities of about 1 to about 15 weight percent of the total oxides. In many instances, catalytic activity may be as great with about l to 5 percent of aluminum oxide as with about 10 to 15 percent. Still greater amounts up to about S0 weight percent may be added, if desired, although the physical characteristics and 4activity of the catalyst may be adversely affected. ln order to retain the selectivity of the catalyst for the present reaction, other heavy metal oxides than those hereinbelow recited, or salts, are usually absent from the starting materials and the finished gel. Oxides ol metals of group IIIB and IVA of the periodic system may be incorporated with the silica and alumina, if desired. For example, small quantities of zirconia may be used in addition to alumina for activating the silica gel. Such metal oxides may be added in the same ways discussed above with respect to aluminum oxide.

Because of the removal of para-xylene in purication zone 32, the xylene mixture charged to isomerization zone 36 is below the thermodynamic equilibrium in paraxylene content. For this reason, a portion ofthe ortlttif: xylene and meta-Xylene in this mixture -is converted vto para-xylene, thus once again approaching the thermodynamic equilibrium in para-xylene content. A small amount of light hydrocarbons, such as methane, ethane, propane, butane and pentane, and heavy hydrocarbons, including C8 and C9 parans, C9 aromatics and C3 naphthenes, produced in the isomerization step, is removed Vfrom isomerization zone 36, together with the desired isomerization product, through conduit 37. It is desired to return this product stream to separation zone 25 through conduits 37, 42, 43 and 26. However, if this cycle were to be indefinitely continued, the build-up of paran, naphthene and other undesired impurities which boil in substantially the same boiling range as the desired xylene fraction, would be such as to eventually cause a shut-down of the operation.

We have devised a process whereby it is possible to utilize this product stream vfor the recovery of the desired para-xylene product Without an undue build-up of undesired impurities. By the process of this invention, light materials are removed as overhead from separation zone 38 through conduit 39. Heavy materials, including the xylenes, are removed through conduit 42. A side stream of relatively light material is removed through conduit 41 to conduit 40. We remove a minor amount, say from 8 to 20 percent, preferably l0 to 15 percent, ofthe stream removed from separation zone 38 through conduit 42, and pass that portion of the stream, together with the stream from conduit 41, into extraction zone 14 through conduits 40 and 44. The remainder of the material removed from separation zone 38 through conduit 42 is passed through co'nduit 43 to separation zone 25. A majorportion of undesired paratlinic impurities contained in the stream passed through conduits 40 and 44 is removed in extraction zone 14 and that purified stream from which the undesired impurities have been removed is returned to separation zone ,25, together with product from hydroforming zone 12. Operating in this manner, we are able to continuously utilize ,the product stream hom isomerization zone 36 without encountering any diiculty whatever by reason of the production of small amounts of undesired impurities.

We operate isomerizatio'n' zone 36 under' 'isomerization conditions which include a temperature-within the range of 600 F. to 1000 F., preferably 700 F. to 900 F., a pressure of from atmospheric to 1000 p.s..g., preferably atmospheric to 500 p.s..g., and a liquid hourly space velocity of 0.3 to 10, preferably 0.5 to 6. Naphthene impurities which are found in the product stream removed from isomer-ization zone 36 generally rult from trace metals which act as hydrogenation or dehydrogenation catalysts and, in the presence of added hydrogen, tend to convert the aromatics to naphthenes. Y Y

Better understanding of this invention will be obtained upon study of the following example which is presented as being exemplary and not with the idea of unduly limiting the scope of this invention.

EXAMPLE A product stream having the following composition is removed from isomerization zone 36.

6 L f; PO'llndS Cgaromatics 22,240 Ca Parans 11,499 C9 parans .11,489 C8 naphthenes 2,970

Total A 578,530

That stream is passed to separation zone 38, maintained at a pressure of150 p.s.i.g.- and a kettel temperature of 450 F., where the light materials, hydrogen through butane, are removed overhead, together with 1800 pounds of the pentane. A bottom'stream from separation zone 38 is divided into two parts, a rst stream having the following composition is passed through solvent extraction zone 14, better known asa Udex separation, using diethylene glycol as the solvent. The extraction is carried out in this extraction zone at a pressure of 60 p.s.i.g. and a temperature of 250 F.

Pounds C-ibenzene ,e- 114 Toluene 1,439 Ethyl benzene '7,960 Para-xylene 7,046 Meta-xylene 17,521 Ortho-xylene 8,570 C9 aromatics 2,224 Cs paraffins 1,148 C9 parains 1,148 C8 naphthenes 297 Total 47,467

A side stream having the following compositionis removed from zone 38 and is also passed to solvent. extraction zone 14.

Pounds Pentane Ce-lbenzene .10,250 Toluene n 14,500 Total 24,750

The remainder of the bottoms stream from separation zone 38 yis passed to separation zone 25, operated ata pressure of 2 p.s.i.g. and a kettle temperature of 3209 F. A fresh feed having'the following composition is added to the system through conduit 13.

P-ounds The following materials are removed by the separation zones 14 and 25.

Pounds Pentane 100 CG-ibenzene 11,390 Toluene 17,390 C9 aromatics 10,740 C8 para'ins 1,150 C9 parains 1,150 C8 naphthenes 270 Total 42,190

The resulting para-xylene concentrate is passed to purification zone 32 through line 29, where it is cooled to a temperature of 100 F. so as to cxystallize out para-xylene.

7 Product Toluene Y. 20 Ethyl benzene 140 Para-xylene 64,140 Meta-xylene 280 Ortho-xyleueV 130 C, aromatica 20 C8 paralus 20 C9 parains 20 Total 64,770

A stream having the following composition is fed to isomerization zone 36, together with 38,200 pounds of hydrogen and 30,400 pounds of methane Pounds Toluene 15,140 Ethyl benzene 105,860 Para-xylene 24,800 Meta-xylene 215,380 Ortho-xylene 107,960 C9 aromatics 15,130 C8 paratins 11,480 C9 parains 11,480 Ca naphthenes 2,700

Total 509,930

Although this specific example has been disclosed as utilizing diethylene glycol as the solvent in the solvent extraction zone 14, other solvents, than diethylene glycol, can be utilized in such a separation to etect the removal of parainic materials from the material which is to be Vsubjected to the purification step. Additional solvents which can be used for separating aromatics from petrole` um mixtures are as follows:

Benzaldehyde I Triethanolamiue Eugenol Diphenyl amine Aeetophenone Xylenol Carbitol acetate Butyl Carbitol Phenetidine Dibutyl phthalate The specific conditions under which extraction zone 14 is operated will be dependent upon the specic solvent utilized, the conditions being such that the solvent and hydrocarbons are maintained as liquids.

It will be apparent to those skilled in the art that various modilications of this invention can be made upon study of the accompanying disclosure. Such modifications are believed to be clearly within the spirit and the scope of this invention.

We claim:

A process for the production of para-xylene from a mixture of isomeric Xylenes which comprises passing said mixture to a crystallization zone; cooling said mixture in said crystallization zone to a temperature whereat paraxylene is crystallized out of solution thereby recovering same; subjecting the mother liquor from said crystallization zone to isomerization in the presence of an isomer-ization catalyst and under isomerization conditions so as to produce a mixture enriched in para-xylene; separating this last said mixture into a low boiling fraction, an intermediate boiling fraction and a high boiling fraction, said high boling fraction comprising isomeric Xylenes; divid-V ing the high boiling fraction into a major and a minor stream; combining the intermediate fraction with said minor stream; subjecting the combined intermediate fraction -and minor stream to solvent extraction to remove paraffns and naphthenes formed in said isomerization step; combining the thus treated combined minor stream with the major stream and passing the combined stream to said crystallization zone along with fresh feed of isomeric xylenes.

References Cited in the le of this patent UNITED STATES PATENTS 2,425,559 Passino et al. Aug. l2, 1947 2,521,444 Brooke et al. Sept. 5, 1950 2,527,824 Kemp Oct. 3l, 1950 2,532,276 Birch et al Dec, 5, 1950 2,564,388 Bennett et al. Aug. 14, 1951 2,656,397 Holzman et al Oct. 20, 1953 OTHER REFERENCES Kalichevsky: Modern Methods of Refining Lubricating Oils (1938), Reinhold, NY., publisher, page 134,

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